1. Field of the Invention
This invention relates to improved magnetic coating compositions and magnetic recording media produced with such compositions, as well as certain novel fluoropolyols.
2. Description of the Prior Art
Audio, video, computer, and other magnetic recording tapes and disks are subject to extensive wear during use. During both recording and playback the magnetic medium is in frictional contact with the read/write head resulting in abrasion of its surface with concommitant loss of signal, read/write head wear, and even audible squeal. Because of such wear the lubrication of magnetic media used in tape, disk, and other applications has played an important part in their development (See R. H. Perry and A. A. Nishimura, Kirk-Othmer: Encyclopedia of Chemical Technology 14, 732-753 (1981)).
A magnetic recording tape or disk consists of a magnetic coating on a flexible or rigid (hard disk) nonmetallic substrate. Historically, these coatings have been made from a suspension of magnetic particles dispersed in an organic binder. More recently, magnetic films produced by direct sputtering of magnetic materials on the substrate have been developed. The present invention relates primarily to magnetic coatings based on pigment dispersion.
Basic manufacture of a magnetic particle based system involves the coating on the substrate of choice with a formulated product containing pigment, organic binder, solvent, dispersants, lubricants, conductivity aids, and other miscellaneous additives. Dispersants are used to deagglomerate the magnetic particles and to provide for their maximum possible separation while maintaining maximum pigment-to-binder volume ratio. Magnetic tape performance is critically dependent on the nature of this dispersion and its volume ratio. Numerous dispersion agents have been used, such as lecithin, organic esters of phosphoric acid, organic acids, quaternary ammonium compounds, organic amines and miscellaneous surfactants. Of these, the more successful have been soya lecithin and synthetic phosphate esters. Lubricants are used to reduce the head-to-tape frictional interaction. Earlier versions and much of the current productions of magnetic tape involve the use of rather well known lubricants: fatty acids and esters, glyceryl esters, fatty acid amides, silicones, hydrocarbons and other refined oils. The selection of lubricant is not necessarily easy. The ideal lubricant must not only be compatible with the formulated coating solution during manufacture, but after cure and dry, must also migrate to the head-tape interface at the proper rate to provide lubricity yet not exude excessively to cause build-up on the head. The more common lubricants used include: isocetylstearate, butoxyethyl stearate, butylmyristate, silicone oils, and synthetic sperm oil.
More recently, various fluorinated materials have been shown to be excellent high-performance lubricants. In particular perfluoroalkyl polyethers have been used on both magnetic pigment/binder and on sputtered magnetic film recording tapes. Because of incompatibilities with other components of the formulated magnetic coatings used for pigment/binder tape manufacture, the fluorinated materials are generally applied as a coating to the previously manufactured tape. This post-treatment represents a costly and unwanted step in the manufacturing process (see U.S. Pat. No. 3,778,308).
The usual perfluorinated fluid or oil is essentially a non-polar material and, as such, has little tendency to bind itself or stick to the magnetic coating surface. When used as a lubricant film on the magnetic coating surface these materials are easily removed and therefore do not provide long term protection for the tape. Because of this, considerable effort has been expended to develop fluorinated materials with associated polar groups (see U.S. Pat. No. 4,696,845). The presence of these polar groups causes the fluorinated fluid to be strongly bound to the magnetic coating surface and to provide longer term protection to the tape. In some cases the non-polar fluorinated fluid can be used in conjunction with a second coating material which enhances its binding to the surface. In this latter case, a second, undesirable, post-treatment step in the manufacturing process is required (see U.S. Pat. No. 4,722,859).
It would be desirable to provide a class of fluorinated materials which overcomes the problems discussed above yet provides long term wear for magnetic coated tape. It would also be desirable to provide a class of fluorinated materials which is an excellent dispersant for the magnetic pigments used and therefore enhances the overall performance of the tape.
Certain fluoropolyols are known in the art. See, for example, U.S. Pat. Nos. 3,720,639; 3,852,222 and 4,284,747. It would be desirable to provide other, novel fluoropolyols which are useful, in particular, in magnetic coating compositions.